Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. Sputtering was largely developed to sputter deposit planar layers of aluminum metallization for electrical contacts. However, in recent years, advanced integrated circuits have increasingly used copper metallization applied by electrochemical plating (ECP) but have also relied upon sputtering for the deposition of thin conformal liner layers in high aspect-ratio holes including barrier layers such as Ta/TaN and copper seed layers for the later applied ECP copper.
Sputtering thin layers into deep holes such as vias has required that a large fraction of the sputtered atoms sputtered from the target be ionized and that the wafer be electrically biased to attract the ions deep within the hole. In a widely used approach, a small magnetron is rotated about the target center near its periphery to project a strong magnetic field adjacent the sputtering face of the target to create a high-density plasma, which not only increases the sputtering rate but also produces a high fraction of ionized sputter particles. Even though the magnetron is located away from the target center, the ions tend to diffuse towards the center and sputter deposit across the entire wafer. In fact, unless other precautions are taken, the sputter deposition tends to be heavier at the wafer center than at the wafer edge because of edge loss of ions to the chamber walls.
The small peripherally located magnetron, however, suffers from the problem that a substantial fraction of the sputtered atoms redeposit on areas of the target that are not being sputtered. The redeposited material at the target center is not subjected to further sputtering and forms a thickening films which does not adhere well to the underlying target. At some point, the redeposited film flakes off the target and generates an excessive number of particles within the chamber. Such particles tend to fall on the wafer being processed and create defects in the resultant integrated circuit with a resulting loss in yield or degraded device reliability. As a result, it has become common practice to occasionally clean the target. In the cleaning mode, usually with production wafers absent from the sputtering chamber, the sputtering conditions are modified so that the center of the target is sputtered to remove the sputter material redeposited there.
The known cleaning techniques have been complicated by the shrinking size of the peripherally located magnetrons and increasing size of the central area of the target requiring cleaning. Any solution, however, should not incur a great increase in chamber complexity or cost or impact system productivity by an excessive cleaning time.